Gas discharge lamps, such as flourescent lamps, are known in the art. It is also known to place an electronic ballast oscillator on the gas discharge lamp to regulate its operation point through regulation of the current flowing through the gas discharge lamp. It is known that, in order not to overstress the filaments of the gas discharge lamp, a preheat time is necessary upon startup. A prior art solution is to place a thermistor at a location where the current through the thermistor roughly equals the current through the load of the gas discharge lamp. The slow thermal time constant of the thermistor will ensure that the voltage received by the electronic ballast oscillator will, upon startup, slowly rise to a high voltage. Filament preheating occurs until the voltage achieves the gas breakdown voltage.
Prior art methods of automatically extinguishing the oscillation of the electronic ballast oscillator utilize an SCR. The SCR can be placed in various locations in the circuit and its purpose is to short the oscillator loop, thereby stopping the oscillations. The SCR is utilized to withstand the high surge currents in the system.
It is known that, as a gas discharge lamp ages, the amount of gas in the lamp decreases, thereby requiring a greater voltage to produce the same amount of light. Overvoltage protection is necessary. In prior art electronic ballast oscillator systems, overvoltage is sensed by sensing the voltage across the lamp and by comparing it to a reference voltage proportional to the input D.C. voltage. If the voltage across the lamp is greater than the reference voltage by more than a predetermined threshold, the lamp is shut off.